Re: [PATCH v2 12/16] pinctrl: starfive: Add pinctrl driver for StarFive SoCs

[Andy Shevchenko]

On Sun, Oct 24, 2021 at 12:29 PM Emil Renner Berthing <kernel@xxxxxxxx> wrote:
> On Sat, 23 Oct 2021 at 23:02, Emil Renner Berthing <kernel@xxxxxxxx> wrote:
> > On Sat, 23 Oct 2021 at 22:29, Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:
> > > On Sat, Oct 23, 2021 at 9:46 PM Emil Renner Berthing <kernel@xxxxxxxx> wrote:

...

> > So is that a yes or a no to my question? It's not clear to me.
>
> I see now that you've probably misunderstood what the code does. It's
> not one time use. The function parses the device tree and dynamically
> registers groups and functions with the pinctrl framework. Each group
> needs a string name, an int array of pins and optionally the pinmux
> data. Once the group is registered those pieces of data needs to live
> with the group until the drive is unloaded. But if the device tree
> parsing fails before the group is registered then those allocations
> would never be referenced and just hang around as garbage until the
> driver is unloaded. In such cases fx. pinctrl-single uses devm_free to
> free them again.

Thank you for elaboration. Please, drop devm_*(). In this case it's
inappropriate to use it. pinctrl-single should be amended accordingly,
but it's out of scope here.

...

> > > > I such cases where you get conflicting PIN_CONFIG_BIAS_* settings I
> > > > don't see why it's better to do the rmw on the padctl register for the
> > > > first bias setting only to then change the bits again a few
> > > > microseconds later when the loop encounters the second bias setting.
> > > > After the loop is done the end result would still be just the last
> > > > bias setting.
> > >
> > > It could be bias X followed by something else followed by bias Y. You
> > > will write something else with bias Y. I admit I don't know this
> > > hardware and you and maintainers are supposed to decide what's better,
> > > but my guts are telling me that current algo is buggy.
> >
> > So there is only one padctl register pr. pin. I don't see why first
> > setting the bias bits to X, then setting some other bits, and then
> > setting the bias bits to Y would be different from just setting all
> > the bits in one go. Except for during that little microsecond window
> > during the loop that I actually think it's better to avoid.
>
> Maybe an example is in order. Suppose we get strong pull-up, drive
> strength 3 and pull-down config flags (the strong pull-up and pull
> down flags conflict) and the padctl value is 0x0c0 (pull-up, input and
> schmitt trigger enabled). With your solution of just altering the
> padctl bits immediately we'd call starfive_padctl_rmw 3 times in rapid
> succession like this:
>
> starfive_padctl_rmw(pin, 0x130, 0x100);
> starfive_padctl_rmw(pin, 0x007, 0x003);
> starfive_padctl_rmw(pin, 0x130, 0x010);
>
> ..and the end result would be 0x0d3, although the strong pull-up would
> be enabled for the microseconds between the 1st and 3nd call.
> As the code is now it'd just directly do
>
> starfive_padctl_rmw(pin, 0x137, 0x013)
>
> ..which again results in 0x0d3, only without the microsecond blink of
> the strong pull-up.

You missed the point. Hardware on the other end may behave well
differently in these two cases.

-- 
With Best Regards,
Andy Shevchenko